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midterm 2 retry part two

midterm 2 retry part two - 1 870 Nitrite an Electron Donor...

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Unformatted text preview: 1 870 Nitrite, an Electron Donor for Anoxygenie Photosynthesis Benjamin M. Griffin,* Joachim Schott, Bernhard Schink lthough compounds of the sulfur cycle, and more recently the iron cycle, are weglw studied electron donors for anoxygonic photosynthesis, no analogous oxidations in the ni~ trogen cycle are known. We report a previously unknown process in which anoxygenic photo— trophic bacteria use nitrite as an electron donor for photosynthesis, providing a microbial mmhunistn for the stoichiometric oxidation of nitrite to niuute in the absence of oxygen. To examine nitrite as a possible electron donor for anoxygcnic photo trophs, we established enrichment cultures derived Nitrate or Nitrite (mM) Fig. 1. Time courses for nitrite consumed (A), nitrate produced (It), cumulative nitrite consumed (6), and growth as the change in optical density (AODW) (0) for triplicate enrichment cultures (N z 3). Data are mean x SD. (A) initially incubated in the light. (8) initially incubated in the dark. The plus signs indicate nitrite feedings, and arrows denote a switch from the initial light mndition. The minus signs indicate when the cultures were starved of nitrite to assess nitrite dependence of growth. (C) Phase—contrast micrograph of strain KS. The scale bar represents 10 um. from local sewage sludge and several firesltwater sediments in anoxic. bicarbonate—bulletin mineral medium (1). Low amounte of nitrite ( l to 2 mM) were fill repeatedly to avoid toxicity, and the cultures were incubated continuously in the light. After incubating in the light for several weeks, enrichment cultures from ll) out of 14 sampling sites oxidized nitrite to nitrate and dc— velopcd pink coloration, as typical ofanoxygenic phototrophs. Absorption spectra of intact cells revealed maximal at 799 um and 854 run, which are characteristic of bacteiiechlorophyll u (2). No chlorophyll a or oxygen was observed in nitrite— 29 JUNE 2007 VOL 315 SCiENCE oxidizing cultures. suggesting that nitmte did not form because of a combination of oxygenic photosynthesis and aerobic nitrification. No growth or nitrite oxidation occurred in cultures incubated in the dark or in uninoculated bottles. thereby ruling out the possibilities that nitrate was produced by anaerobic ammonia oxidation (attainmox) or abiotic, photochemical processes. Lightndnrk shift experiments perfoimed over several days with enrichment cultures transferred five times showed that growth and nitrate produc— tion depended on both light and nitrite (Fig. l). 00666 The rate of nitrite consumption increased on mul_ tiple feedings and approached 2 mM per day after i week in the light. As expected for a photonuto» trophic process, nitrite eonswned, nitrate produced, and biomass formed were all tightly comalated; ni— trate was limited from nitrite neetr stoichiomeuically. We isolated the numerically dominant coccus (2 to 3 um in diameter) from the most active en- richment culture derived from Konstanz sewage sludge by dilution to extinction in liquid medium (Fig. 10) (1). Analysis of the MS ribosomal RNA gene sequence revealed that the strain, dose ignored KS. is most closely related to Thiocapsa roscopenricirm (98% identical). Thioenpsa spe— cics are widely distributed purple sull‘or bacteria of the order Chromatialcs and are metabolic get» enlists capable of photoautotrophic growth on a variety ofcomrnon inorganic electron donors, in ad— dition to aerobic chemolithoaulotrophic growth (3). Although photoimphs are known to directly influence the nitrogen cycle through teductive processes such as nitrogen fixation, assimilation, and respiration (4), this is the only example of a photosynthetically driven oxidation in the nitro- gen cycle. in principle, this photosynthetic pro- cess could compete for nitrite in the environment with other key nitrogen cycle processes such as denitrilicntion, aerobic nitrification, or anammox. In 1970, Olson proposed in detail how the wateroxidizing activity ofoxygenic photosyntho sis may have evolved from anoxygenic photo~ synthesis through a series of inorganic nitrogen electron donors with hlmoing midpoint potentials (.3). The ninite~nitmte couple, with a standard redox potential of +0.43 V, could theo— retically donate electrons to the quinone—typc reaction center in pur— ple sulfur bacteria, when: the hac- teriochlorophyll primary donor has a midpoint potential as high as +0.49 V (6). This work demon,— stretes nitrite as the highest-potential electron donor for nnoxygenic photow synthesis loiown so far and provides a modern example of an electron donor once implicated in the evo— lution of oxygenic photosynthesis. References and Notes 1. Materials and methods are available on Scienre Unline. 2. i. F. lniholt, in Anoxygenic Photosynthetic Bacteria. R. E. Blankenship, M. l. Madigarz. C. E. flaunt, Eds. (Kluwer, Bordrechi, Netherlands, 1995), pp. 3:15. 3. ]. F. lmhoff. in fire Prokaryotes, M. Divorkin et al., Eds. (Springer, Verlag, New York, ed. 3, 2006), vol. 6, pp. 846~~873. 4, l. 9. Megonigal, M. E. Hines, P. T. Visschor, in irenn'se on Geochemistry, vol. 8, W. H. Schlesinger, Ed. (Elsevier, Amsterdam, 2003), pp. 317—424. 3. M. Olson, S'Cienre 168, 438 (19.70). 6. M. A. Cusanow‘th, R. G. Baruch, M. D. Kamen, Biochim. Biophys. Ada 153, 397 (1968). Supporting Online Material www.sciencemagergkgiicontenMull/316/5833f1570/9C1 Materials and Methods References in 3 ianuaty 2007; accepted 19 April 200'? 10.1126/9ciente.1139478 Department for Biology, Universitét Konstanz, D7845] Konstanz, Germany. *Present address: institute for Genomic Biology, University of lllinois, Urbano, it 51801, USA. To whom correspondence should he addressed. E‘mail: [email protected] vwrwsclencemagorg .uv Downloaded from want! Reflex coup“ , w PSIWM (”038) “ _ ' _ " mm m (mama (mm («032) (”x/m3 Ismfiwiugs “9.221 a a“ 2Wmmwfiactate («43.19; 2 as" 542%?“526333” («3.9242 2 «a» UNEXCIT€E> P8115 OX/red {W} GHQ) mm‘”f§%mz gimm 5 e N " FWM“ :mm 1 a {5er 2%) M” “ gm” (+£3.32; 2 as“ W; Gm mm Ks. Mfiwm Mmgamm NP: M E: mmfimm my HE gm 35$ ...
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